The invention relates to a method of mixing chemicals to produce a sol-gel for subsequent application to a metal substrate. More particularly, the invention relates to an automated method of mixing raw chemicals to produce a ready to apply sol-gel solution.
Various coatings have been developed which promote the adhesion of one or more resinous layers to a titanium, aluminum, or other metallic surface such as that of an airplane component. Some of the coatings improve adhesion by utilizing a sol-gel film between the metal and the resin. The term xe2x80x9csol-gelxe2x80x9d, a contraction of solution-gelation, refers to a series of reactions where a soluble organometallic species, typically a metal alkoxide or metal salt, hydrolyzes to form a metal hydroxide.
The sol-gel films or sol coatings promote adhesion through a hybrid organometallic coupling agent at the metal surface. The metallic portion of the organometallic, which usually contains zirconium, bonds covalently with the metal. The organic portion of the organometallic bonds covalently with the applied layer of adhesive or matrix resin. In this manner, the organometallic based sol-gel generates a bridging structure that creates a chemical link between the metal surface and the resin primer.
The strength and durability of the sol coating depends upon chemical and micro-mechanical interactions at the surface of the metal involving, for example, the porosity and microstructure of the metal and the tendency of the sol coating to rehydrate. When properly implemented, the sol-gel coatings provide surface stability for paint adhesion, adhesive bonding, or fabrication of structurally superior fiber-metal hybrid laminates.
A sol-gel composition that is particularly useful for coating aluminum and titanium surfaces is based on a combination of organometallic and organosilane components. The preferred organometallic compound for use in a sol-gel for coating aluminum and titanium surfaces is an alkoxy metallic compound, and more preferably an alkoxy zirconium compound. Because of its ready commercial availability, Zr (IV) n-propoxide is particularly preferred as the organometallic compound. In addition to covalently bonding to the metal surface, the organozirconium compound also serves to minimize the diffusion of oxygen to the surface and to stabilize the metal-resin interface. Epoxy-functionalized silanes are the preferred organosilanes because of their stability in solution and their ability to crosslink with common, aerospace epoxy or urethane adhesives. The silane is acid-base neutral, so its presence in the sol mixture does not increase the relative hydrolysis and condensation rates of the alkoxy metallic compounds. Sols including the organosilanes are relatively easy to prepare and to apply with reproducible results.
A particularly useful sol-gel formulation is Boegel-EPII(trademark), developed by The Boeing Company, Seattle, Wash. The Boegel-EPII(trademark) composition is a combination of 3-glycidoxypropyltrimethoxysilane (GTMS) and Zr (IV) n-propoxide which is reacted in the presence of an acetic acid stabilizer. The GTMS has an active epoxy group which can react with common epoxy and urethane resins. GTMS does not form strong Lewis acid-base interactions with the hydrated metal oxide substrate. The zirconium in the mixture tends to react more quickly with the oxide surface of the metal, allowing the desired stratification of the sol-gel film with the epoxy groups of the silane coupling agents oriented toward the resin layer.
To achieve the desired coating structure, it is desired to apply the sol-gel solution to the substrate at the point that the zirconium and silicon are hydrolyzed sufficiently that zirconium and silicon react with the metal surface. If the sol is applied too shortly after beginning the reaction between the organometallic and organosilane, the organosilane may not be fully hydrolyzed upon application. If the sol is not applied soon enough, the hydrolyzed silicon and organometallic components may condense among themselves, forming oligomers and networks.
The need to properly prepare and age the sot-gel before application to a substrate can present a problem because the need for aging causes delays in the coating process. Also, sol-gel that is over-aged must be discarded as waste. Appropriate timing of mixing can increase the efficiency and cost effectiveness of the sol-gel coating process.
It is, therefore, desired to provide a method and apparatus which reacts the components of sol-gel without the need to manually monitor the aging or mixing of the sol-gel. It is further desired to provide an apparatus which supplies sol-gel on demand so that over-aged sol-gel need not be wasted.
A sol-gel mixer is provided according to embodiments of the present invention which provides a properly mixed and properly aged sot-gel mixture for application to a metallic surface. The sol-gel mixer is automated such that a series of valves, pumps, and tanks control the reaction and mixing of the sol-gel. Flow rates and dwell times are controlled so that aged sol-gel is available from the mixing process such that the sol-gel may be generated on an as-needed basis.
The inputs for the sol-gel mixer comprise a supply of solvent, preferably deionized water, a supply of acid, preferably glacial acetic acid, a supply of zirconium alkoxide, preferably zirconium n-propoxide (TPOZ), and a supply of organosilane, preferably 3-glycidoxypropyltrimethoxysilane (GTMS). A series of controlled valves and pumps first supply the acid and zirconium alkoxide to an agitated pre-mix vessel where the components are allowed to dwell. A supply of solvent is then combined with the output from the pre-mix vessel, which is then combined with a supply of organosilane and solvent in an agitated final-mix vessel. The mixed sol-gel is held for a given induction time and then stored until ready for use. The aged sol-gel is preferably supplied directly from the output of the final-mix vessel.
Each step in the mixing process is automated and is regulated by a controller, preferably a personal computer (PC) or other processing element. Through mechanical interfaces, the controller switches valves and actuates pumps such that precise, predetermined volumetric amounts of acid and zirconium alkoxide are combined and mixed within the pre-mix vessel and so that precise, predetermined amounts of solvent and organosilane are combined with the pre-mixture in the final-mix vessel. Dwell times of the components within the vessels are also automated by the controller.